Vision systems have become ubiquitous in modern society. They are used in a wide array of applications including security, safety, industrial control, communications, inventory management, etc. For most applications, vision systems sensitive to visible light are sufficient. Increasingly, however, vision systems sensitive to the infrared spectrum have become desirable. Infrared visions systems are particularly attractive for applications that require imaging in low light situations (e.g., night vision systems), through foliage or other similar obstructions, and in partially obscured atmospheres such as fog- or smoke-filled air.
Some infrared cameras are sensitive to electromagnetic radiation having wavelengths in the range of 0.8 to 0.9 microns, often referred to as the “near-infrared” (NIR). Conventional NIR vision systems include traditional night vision goggles. Night vision goggles typically employ a photocathode, a microchannel plate, and a direct view phosphor to increase the intensity of low light level images. Although they can provide good low-light-level image quality, such image intensifiers are mechanically complex systems based on vacuum tube technology. In addition, image intensifiers typically only provide an ephemeral image directly on a phosphor screen and do not enable the capture of an image for storage or transmission.
At longer wavelengths of light, such as 8-10 microns (i.e., “long-wave infrared” (LWIR)), infrared cameras typically employ elements that are sensitive to energy based on the temperature of an object, such as microbolometers. A microbolometer is an extremely small heat sensor that comprises materials such as vanadium-oxide or amorphous silicon. The electrical resistance of these materials changes in response to received LWIR radiation. Microbolometers, however, exhibit self-heating effects, poor sensitivity, and generate electrical noise. Moreover, because they are sensitive only to thermal radiation, an object is detected only by the difference of its temperature from that of the background. This can result in poor image resolution.
Photodiode-based infrared cameras have been developed to overcome many of the limitations of other infrared imaging sensors. Such imaging sensors typically utilize a two-dimensional photodiode array comprising photodiodes that are sensitive to infrared radiation. Each photodiode provides an electrical signal based upon the intensity of the infrared radiation that is incident upon it. The photodiode array is electrically coupled to a read-out integrated circuit (ROIC), which includes circuitry for processing each of the electrical signals provided by the photodiode array. The photodiode array is typically disposed at the focal point of a camera lens, and the photodiode array and associated ROIC collectively define a “focal plane array (FPA).”
For imaging in the short-wave infrared (SWIR) region of the optical spectrum (i.e., 1 to 1.7 micron), photodiodes based on the InGaAsP material system have emerged as a preferred device technology. Photodetectors with a positive-intrinsic-negative (p-i-n) structure based on this material exhibit low noise at room temperature. However, for low-light-level imaging in which single photon sensitivity is the ultimate goal, existing image array pixels with state-of-the-art p-i-n detectors and amplifiers can not provide imaging with single photon sensitivity. Moreover, to obtain acceptable quality images in low-light-level conditions, these pixels must integrate photoelectrons generated by the incoming signal for fairly long periods of time (e.g., 100 ms or longer).
Almost all imagers available today are based on analog detection mechanisms. Generally, photodetectors are employed to generate photocarriers that are accumulated at the pixel level to build up an analog signal corresponding to an incident optical image. CMOS imagers and CCD imagers are examples of analog imagers. The sensitivity, or signal-to-noise ratio, of an analog pixel is degraded by both the electrical and thermal noise that originates in both its photodetector and its associated read-out electronics. Even in the absence of any input light, electrical noise is generated in the photodetector due to the shot noise associated with a continuous finite leakage current (a.k.a., dark current). The dynamic range of these imagers is also directly degraded by this leakage current.
Approaches for overcoming limitations of prior-art analog imagers include digitization of the image data at the pixel level One such approach provides digitization functionality within each pixel of the ROIC through the implementation of appropriate pixel-level circuitry, such as a sigma-delta circuit. However, this approach does not obviate the inherent analog nature of the photodetectors used with it.
Other approaches have included two-dimensional arrays of single-photon avalanche diodes (SPADs). These prior-art SPAD approaches have found limited success for low-light-level passive imaging, however. While it is relatively straight-forward to initiate an avalanche event in response to a received photon, it can be difficult to stop it once it starts. SPADs are capable of generating easily detected currents by biasing the avalanche diode above its breakdown voltage. When operated in this fashion, a single photoelectron can give rise to a self-sustaining avalanche current. The utility of this current is that it is easily detectable, but following threshold detection, this current must be “actively quenched” by proactively lowering the applied voltage below the breakdown voltage. Once quenched, the device is subsequently re-armed by again raising the bias above the breakdown voltage. This sequence of “arm, avalanche, active quench, and re-arm” in SPAD operation requires appropriate pixel-level circuitry in the ROIC. Although this circuitry is considerably simpler, lower power, and more space-efficient than the circuitry required for analog pixel operation, it still adds complexity to the operation of the detector at each pixel. In addition, the response rate of such a pixel can be limited by the speed of the active electronics used to quench the photodiode.
The probability that a detection event occurs in response to a photon arrival at the detector is known as the photon detection efficiency (PDE). The probability that a detection event occurs when no photon arrives is known as the dark count probability, and the number of dark counts per second is the dark count rate (DCR). (Dark counts occur when carriers are created by processes other than photo-excitation—e.g., thermal excitation or tunneling processes—and are analogous to dark current in analog photodetectors inasmuch as the fluctuations in the DCR contribute shot noise to the overall detection process.)
While prior-art SPADs do provide single photon sensitivity, they are non-ideal with respect to other performance attributes. The avalanche process gives rise to a large number of carriers traversing the device layer where avalanche gain occurs, and some fraction of these carriers can be trapped at defects present in this layer. Gradual detrapping of these trapped carriers can lead to additional spurious detection events if the detrapping occurs when the device has re-entered its armed state. These additional spurious counts are referred to as “afterpulsing”. If the device is held in its quenched state until all trapped carriers have been detrapped, then afterpulsing can be avoided. However, the introduction of a long “hold-off time” before re-arming the device limits the repetition rate at which photons can be counted. Prior art InP-based SPADs have required hold-off times on the order of 1 μs or more, limiting count rates to the order of a few MHz.
An FPA that provides single photon sensitivity with image data that is digitized at the pixel level, and that is capable of high-dynamic range video rate operation, would be an advance over the prior art.